Mutations and Horizontal Gene Transfer Flashcards
Proofreading Enzymes
Enzymes that can back up and excise a nucleotide not properly hydrogen bonded to the opposing nucleobase in the template strand
Mismatch Repair - Function
Fixes errors missed by the proofreading of DNA polymerases
Mismatch Repair - Steps
1 Protein binds to the mismatched nucleobase
2. enzyme cuts the sugar-phosphate backbone
3. enzyme degrades that region of the DNA strand
4. DNA polymerase and ligase adds correct nucleobases to the DNA strand
How does the enzyme know which strand is the strand with the base error?
Template strands are old and therefore have been methylated but newly created replica strands are not methylated after they are first made and that allows the new enzyme to know which strand has the base error
Base Excision Repair - Function
Fixes nucleobases that have been damaged or oxidized
Base Excision Repair - Steps
- DNA glycosylase removes the damaged nucleobase from the sugar-phosphate backbone
- Enzyme recognizes the missing nucleobase and cuts the strand at that site
- DNA polymerase and ligase repairs and fills the strand of DNA
Photoreactivation
relies on an enzyme that uses the energy gained from visible light to break the thymine dimer covalent bonds
What are the methods to repair thymine dimers?
Photoreactivation and Nucleotide excision repair
What is the difference between nucleotide excision repair and base excision repair?
Nucleotide excision repair repairs helix-distorting lesions while Base excision repair repairs non/minimal helix distorting lesions
SOS Repair - Function
Bacteria Only
Last-effort attempt to repair extensively damaged DNA. Repairs DNA that is too damaged to be repaired by other repair mechanisms
What is the main flaw in SOS repair?
SOS DNA Polymerase has no proofreading ability and will continue to make errors in the repair attempt
SOS Mutagenesis
Errors made as a result of SOS DNA Polymerase errors/non-proofreading
Insertional inactivation
Transposons disrupts the function of the gene
Base Analogs
Molecules that structurally resemble nucleobases but have different hydrogen-bonding properties
Intercalating Agents
Flat molecules that intercalate between adjacent bases of DNA and pushes nucleotides apart thereby increasing the chance that insertions or deletions will be made during DNA replication
UV Radiation Muatations
Causes covalent bonds to form between adjacent thymine nucleobases and thereby promoting thymine dimers
X-Ray induced Mutations
Single- and double- strand breaks in DNA
Changes to nucleobases
Double-strand breaks often result in deletions that are lethal to the cell
What are the 3 methods of Horizontal Gene Transfer
Bacterial Transformation
Transduction
Conjugation
Bacterial Transformation
When “naked” DNA is taken up from the environment into the cell
Transduction
DNA is transferred from one bacterial cell to another via bacteriophage
Conjugation
DNA transferred during cell-to-cell contact
Replicon
When transferred DNA has a origin of replication
Homologous Recombination
When a chromosomal fragment is transferred, it then must become integrated into a replicon to be maintained in a population
Types of replicons
Plasmids and Chromosomes
Types of non-replicons
Fragments of chromosomal DNA
What is the origin of “naked DNA”
Lysed Cells whose chromosomes break up into many smaller pieces
What is DNase?
an enzyme that degrades DNA
Competence
a state that allows a cell to take up DNA
What are the steps of Bacterial Transformation?
- donor dsDNA binds to a extracellular receptor on competent cell
- one strand of dsDNA enters the cell, complement stand degrades
- homologous recombination integrates donor cell into recipient’s genome, replaced strand is degraded
- Since only one strand contains the donor DNA, only one of the daughter cells will contain the mutant genome, the other will remain the same as the parent
Plasmid Transfer - Method
Most frequently transferred to other cells via conjugation
Conjugative plasmids
Direct their own transfer from donor to recipient cells
F Plasmid
Fertility plasmid of E. Coli
